The present invention relates to ball screw and nut mechanisms wherein helical grooves are formed in the nut and screw to form raceways for load bearing and recirculation balls, and crossovers are provided as ball return elements.
Ball screws are mechanical devices widely used to convert rotary motion of either the nut or the screw into linear motion of the other. Matching helical grooves in the screw and nut form raceways within which balls roll when either the nut or screw are rotating. As the balls roll in response to this rotational motion they reach one end of the assembly and must be recirculated in a continuous path towards, but not necessarily all the way to the opposite end of the assembly, forming a closed circuit. In order to achieve this recirculation of the balls in the system, there are generally two types of return systems; external and internal. External ball return systems generally involve external ball return tubes which protrude above and around the outside diameter of the ball nut. Internal ball return systems generally involve the ball returning through or along the nut wall, but below the outside diameter. This invention relates to an internal ball return system.
There are several variations of internal ball return systems. In one variation, sometimes termed a “flop-over” design, the ball is forced to climb over the crest of the thread by the return system. In this system, the balls make often less than a full revolution of the shaft and the circuit is closed by a ball deflector that allows the balls to cross over adjacent grooves or thread crests. These ball deflectors often comprise a separately manufactured component, or crossover block, that is inserted, either from the internal diameter or the external diameter of the ball nut, through an opening in the ball nut. This machined slot or bore may have additional retention features incorporated to aid in the retention of the crossover block or may be simply be a straight-walled machined bore or slot, relying on retention features in the deflector. The deflector, whether inserted through the internal surface or the external surface of the ball nut, contains a ball pathway or channel through the center portion of the deflector, often curved in such a way and with a large enough radius such that balls may move over the aforementioned thread crests and recirculate into an adjacent ball raceway.
In another variation of the internal ball return system, the balls are returned to the opposite end of the nut through the nut wall utilizing a hole, or along the nut wall utilizing a channel, still below the outside diameter of the ball nut.
The crossover block of an internal ball return system may be retained in the opening of the ball nut with a press fit or a number of other retention features, including adhesives. If inserted from the interior of the ball nut, this retention may include a step feature towards the outer surface of the ball nut, such that the block is prevented from exiting the ball screw assembly through the hole or slot in the ball nut during operation. In addition, where the crossover block is inserted from the interior of the ball nut it is often only with a slip fit so that the block is retained in place by the constant recirculation of balls in the ball screw assembly. This arrangement allows for some minor movement of the crossover block while the balls recirculate, in turn, possibly causing noise and vibration. In addition, with these movements of the crossover block, the ingress/egress of the balls through the crossover block ball channel changes, possibly impacting ball velocity and causing ball to ball noise and friction.
The principle requirement of retaining such a crossover block within the hole through the ball nut is that the block be aligned properly, such that the ball threads align with the ball channel in the crossover block in order to provide an unobstructed path for recirculation of the balls. For that reason, it is known to locate the crossover block in relation to the interior surface, rather than an exterior surface or step, of the ball nut even where the block is inserted from the exterior of the ball nut, as it allows for less variation and better alignment of the crossover block channel and the ball threads. An example of such an arrangement is disclosed in U.S. Pat. No. 5,937,700, wherein tabs are formed on compressible arms that are pushed inwards during assembly and extend outwards once the tabs align with the ball raceways of the ball screw assembly. This arrangement is necessarily a slip fit, retained mainly by the constant recirculation of balls within the ball screw and through the crossover block channel.